1. Field of the Invention
This invention relates to an improved method for preparing alkyl polyglycosides which includes the reaction of glucose and a fatty alcohol. More specifically, the present invention relates to a method for preparing alkyl polyglycosides which utilizes recycled fatty alcohol wherein elevated levels of interfering contaminants are periodically removed from the recycled fatty alcohol, and recycles the purified fatty alcohol.
2. Description of the Related Art
Alkyl polyglycosides are conveniently prepared by reacting an alcohol of the type and chain length which is desired, usually a fatty alcohol from which is derived the "alkyl" portion of the glycoside of interest, with a saccharide reactant (e.g., a monosaccharide such as glucose, xylose, arabinose, galactose, fructose, etc., or a polysaccharide such as starch, hemicellulose, lactose, maltose, melibiose, etc.) or with a glycoside starting material wherein the aglycone portion thereof is different from the alkyl substituent desired for the ultimate alkyl polyglycoside product of interest. Typically, these reactions are conducted at elevated temperatures and reduced pressures and in the presence of an acid catalyst and using a 2-10 molar ratio of fatty alcohol to monosaccharide. After the reaction has been terminated, usually by the addition of base, the excess fatty alcohol is reclaimed from the alkyl polyglycoside product by removing it by distillation under reduced pressure. The reclaimed, distilled fatty alcohol typically contains various types of co-distilling impurities, some of which were generated in the alkyl polyglycoside reaction and others which entered the process as impurities in "virgin" alcohol. Examples of the former include fatty esters of formic and levulinic acids (solvolysis products of the well-documented saccharide dehydration product HMF, or 5-hydroxymethyl furfural), as well as lactate and acetate esters and various ethers. In addition, other non-alcohol components are found in reclaimed alcohol which can be traced to impurities found in "virgin" fatty alcohols. The concentrations of some of these impurities are seen to diminish after the glycosidation reaction. Examples of these "reactive" impurities are fatty aldehydes, which disappear by acetalization reactions during the glycosidation step. The concentrations of other, so-called "inert" impurities present in "virgin" alcohols are found to increase each time the reclaimed alcohol is reused in the glycosidation reaction. The majority of these impurities result from the manufacturing process or storage of the alcohols themselves. The exact chemical nature of such impurities depends upon the raw materials and manufacturing processes used, and storage conditions. Examples of by-product impurities formed during the manufacturing of "naturally"-derived alcohols are hydrocarbons having 3 and/or 4 more carbon atoms than each of those in the fatty alcohols used, and various ethers having molecular weights comparable to the aforementioned hydrocarbons. The use of repeatedly recycled fatty alcohol which contains a build-up of inerts and other impurities in the process for making alkyl polyglycosides results in a product having less than desirable color and composition and a lower overall product yield.
Therefore, it is an object of the present invention to provide a process for making an alkyl polyglycoside by reacting a sugar and a fatty alcohol to obtain an alkyl polyglycoside and unreacted fatty alcohol reaction product wherein the unreacted fatty alcohol is then separated from the alkyl polyglycoside and recycled. Periodically, the recycled fatty alcohol is substantially purified by removal of all non-hydroxylic impurities, including esters, ethers and hydrocarbons.